The hypothesis is that the fetal hemoglobin (Hb F) program can be modified in erythroid progenitor cells, and that such modification might be of therapeutic benefit in patients with sickle syndromes. Modifiers of Hb F might influence erythropoiesis per se, or might modulate Hb F independently. The long term goal is to increase Hb F in vivo in patients with sickle disorders. The short term goal is to understand differences between normals (Hb AA) and sickle patients (Hb SS) with regard to regulators of erythropoiesis and of Hb synthetic patterns. The specific aims are: 1. To identify growth factors or manipulations which influence in vitro erythropoiesis and/or g globin synthesis in progenitors from normal and sickle patients. 2. To investigate differences in erythropoiesis and Hb F in patients, examining the relative roles of accessory cells and progenitors. 3. To purify erythroid progenitor cells and examine globin synthesis regulation directly. I Mononuclear cells and enriched cell populations will be cultured in semisolid (methyl cellulose) and in liquid systems, and progenitor-derived colonies and cells counted. Globin chain synthesis will be examined by Triton acid urea gel electrophoresis following incubation with 3H-leucine. Culture variables to be investigated include time, oxygen tension, hematopoietic growth factors (including hemin, IL- 3, GM-CSF, SCF, and others), and the relative roles of accessory cells and pure progenitors. Results will be compared for normal controls and patients with Hb SS with high or low Hb F. Media conditioned by accessory and/or progenitor cells will be examined directly by immunoassays for the presence of known growth factors, and indirectly for new factors by examining their role in increasing colony growth and/or g globin synthesis when added to enriched progenitors. Progenitor cells will be purified using monocyte removal by adherence to plastic or iron phagocytosis, T-depletion by rosetting, and positive selection with monoclonal antibodies (CD34). BFU-E from normal controls and sickle patients will be characterized regarding response to growth factors, growth factor receptors, and surface antigens.